Method of manufacturing chemical pulp from lignocellulose material



United States Patent Ofifice 3,088,861 Patented May 7, 1963 METHQD OF MANUFACTURING CHEMICAL PULP FROM LIGNOCELLULOSE MATERIAL Samuel C. McKee, Longview, Wash, assignor to Weyerhaeuser Company, Tacoma, Wash, a corporation of Washington No Drawing. Filed Aug. 3, 1960, Ser. No. 47,140

7 Claims. (Cl. 162- 86) This invention relates to the production of pulp from lignocellulose materials and, more particularly, to a pulping process in which magnesium bisulphite is used as the delignifying agent in the cooking liquor.

In the manufacture of paper from wood pulp optimum commercial practices require a large scale continuous operation of paper-making equipment. The machine speed and product output depend to a large extent on the rate at which the pulp slurry releases its water in the screening operation. This is generally referred to as the drainage characteristic of the pulp and is indicated by its freeness measurement. As is well-known in the art, unbeaten pulp will have a high freeness. This is reduced by beating, and the decrease in freeness is directly proportional to the beating time. Based on readings of a Schopper-Riegler freeness tester, a normal freeness for many paper-making operations is about 700 ml. In order to attain this freeness by the conventional magnesium base sulfite pulping method about 50 minutes heating time are required. Any reduction in the beating time required to produce a desired freeness value represents a saving to the paper manufacturer in capital investment and operating costs of beater equipment. It is, therefore, one object of this invention to improve the magnesium base sulfite process so as to produce a pulp capable of obtaining the desired freeness in a reduced beating time.

As is also known in the art, the low freeness of beaten pulps is due to the fact that the pulp becomes fibrillated upon beating. At extreme the pulp will lose its shape and become an amorphous, gelatinous mass. Consequently, there is a point beyond which additional beating will not produce improved results. Tensile and burst strength will increase with the degree of fibrillation produced by the beating up to a certain point and then decrease. A pulp which will reach the highest tensile and bursting strength in the shortest time of beating is, therefore, most desirable.

Another objective of this invention, therefore, is to so improve the magnesium base sulfite pulping process to produce a pulp which will attain a high tensile and bursting strength in a shorter heating time than conventional magnesium base sulfite pulps.

Two-stage pulping methods have been developed for the soda base process. Such a process has been described in US. Patent No. 2,885,317. In this process the wood is sulfited in the first step with an aqueous solution containing sulfite and/or bisulfite ions and alkali and/or alkaline earth metal ions. The temperature is augmented continuously or step-wise to the desired level during the sulfiting period. The sulfited wood is then hydrolyzed with an acid agent such as sulfur dioxide in the second stage. The proposed theory for this type of pulping is based on a first stage maximum sulfonation of lignin to form an insoluble lignosulfonic acid with a minimum dissolution of wood substance followed by a second-stage hydrolysis or solubilization of the solid lignosulfonic acid to form a soluble product with a minimum dissolution and degradation of wood carbohydrate. The process of the said US. Patent 2,885,317 was developed specifically for the purpose of effecting the pulping of resinous wood of the Pinus species by the sulfite process. It had been found that the phenolic sulfite pulping inhibitors in pine, such as pinosylvin, are sulfonated and made inefiective by carrying out the digestion in a first stage at a pH range of 47. However, long digesting periods were required necessitating the application of high hydraulic over-pressures according to the method of this patent.

It has now been discovered that in applying such a twostage sulfiting process to the pulping of other species and, in particular, nonresinous species such as hemlock, the desired properties of high yield, high hemicellulose content, and rapid development of tensile and bursting strength are largely determined by the first-stage cooking conditions. Particularly it has been discovered that an initial pH in the range of 5 .9 to 5.0 in the first-stage produces these desired pulp properties in the shortest cooking time. The results of a series of cooks wherein soda base sulfite first-stage cooking liquor having different pH values were used to digest hemlock wood chips are compared in Table I below. Other first-stage cooking conditions and the second-stage conditions were the same for all cooks. The first stage temperature was 140 C. minutes to 140 C. and eight hours at 140 C.). The second stage temperature was C. using aqueous S0 solutions.

In determining pH values a liquor sample is Withdrawn from the digester, cooled, and the pH measured at 25 C.

1 Lbs/sq. in./l00 lbs./24 X 36480 ream.

As noted from the figures listed above, in the pH range of 5.9 to 5.0 there is a rapid increase in the bursting strength characteristics (Mullen) with an appreciable reduction in cooking time with only a slight reduction in overall yield. The above values were determined in the same manner as herein later described for determining the values of Table II.

As the economics of pulp manufacture make it desirable to recover the chemicals in and the heat value of the cooking liquors with the elimination of pollution conditions, it would be advantageous to use this improved two-stage sulfite pulping method with a magnesiumbase cyclic sulfite process which has proved commercially successful for recovering the chemicals of cooking liquors.

However, magnesium-base sulfite cooking liquors normally precipitate magnesium monosulfite if the molar ratio of the sulfur dioxide to magnesium oxide is reduced below about 1.85 (corresponding to a pH of about 5.0). Variations in solution temperature and concentration will affect the solubility of magnesium monosulfite, but generally in the normal range of these conditions for commercial cooking liquor preparation this solubility limitation of magnesium monosulfite prevents the preparation of a low S0 to MgO molar ratio, high initial pH first-stage cooking liquor required to provide pulps with these desired characteristics.

I have now discovered that the precipitation of magnesium monosulfite from a magnesium-base sulfite pulping liquor having a low S0 to MgO molar ratio and a high pH can be prevented by preparing the said low molar ratio magnesium-base sulfite liquor in the digester in situ with the wood chips or other lignocellulose material to be pulped.

In accordance with this invention either dry magnesium oxide or magnesium hydroxide, or an aqueous slurry thereof is added to the lignocellulose material in the digester, before, simultaneously therewith, or after the addition of magnesium bisulfite or soluble magnesium sulfite-bisulfite liquor and at the beginning of the sufficient to react with the soluble magnesium sulfitebisulfiteliquor to provide an initial soluble cooking liquor containing as-low as 1.40 moles of sulfur dioxide per mole ofmagnesiurn oxide resulting in an initial pH of about 5.9 without'any observable precipitation of mag-- nesium monosulfite. A preferred molar ratio for thepractice of this method is in the range. of about 1.60 to- 1.70. As there is a tendency for the pH todrop during: the cook due to the generation of acids, it may be desir-- able to increase the initial addition or to make further additions of magnesium oxide or hydroxide during thefirst stage-cooking to maintain the desired pH of the cooking liquor.

This feature of the invention thus comprises a proc-- ess for preparing, in the presence of lignocellulose rna-- tcrial in a digester, a first-stage magnesium-base sulfitecooking liquor having a low molar ratio of sulfur dioxide to magnesium oxide and an initial pH in the range: of 5.9 to 5.0, cooking said lignocellulose material in thepresence of said liquor until to 30% of the former is dissolved, removing most of the first-stage cooking: liquor- (normally an amount in the range of about 60%. to 65% is removed), adding suflicient excess of sulfurdioxide to the digester to lower the pH to approxi mately 1.5, and-cooking said lignocellulose material to complete the desired digestion.-

For the purpose of further illustrating the invention the following example describes a typical cycle of op erations.

Example I A magnesium-base sulfite cooking acid having a total sulfur dioxide concentration of 3.72 g./ 100 ml., and a combined sulfur dioxide concentration of 1.82 g./ 100 ml., was charged to a digester containing hemlock wood chips. Magnesium oxide was added directly to the digester in an amount resulting in the cooking liquor havingan effective combined sulfur dioxide concentration of 2.25 g./100 ml. The ratio of liquor (exclusive of the'water in the wood) was 4.3 pounds per oven dry pound of Wood and provided, on a dry wood basis, a total sulfur dioxide content of 16.0% and a combined sulfur dioxide content of 9.7%.

The addition of the magnesium oxide to the digester provided a cooking liquor havinga molar ratio of sul-.

fur dioxide to magnesium oxide of 1.64 and a pH of 5.4. The temperature of the digester contents was gradually raised from 100 C. to 150 C. in a period of two hours and held there for a period of about four hours.

About 63% of the first-stage cooking liquor was then removed and an aqueous sulfur dioxide solution having a sulfur dioxide concentration of 6.7 g./ 100 ml. was added to the chips to provide a total liquor ratio of 4.3 based on the original wood charge. The second stage cook was carried out at a maximum temperature of 130 C. for two-hours and forty-five minutes at which time the digester pressure was relieved and the contents discharged.

The above temperatures and duration of digesting may be varied depending on the species, types of digesters, and the variations in pulp characteristics that may be desired, all according to the knowledge generally available in the industry.

Pulp, prepared as described in the above example, had characteristics similar to those prepared using the soda base liquors of the desired pH range mentioned hereinbefore in Table I and gave a higher yield than that normally realized from a conventional magnesium-base sulfite pulping process. Further comparisons of the twostage and conventionalmagnesium-base type pulp were made according to the procedures prescribed in the,

TAPPI Standard Methods for Testing Pulp, No. T-ZOO-m45. Following these methods, pulp slun'ies of 1.57% consistence were prepared in a standard Valley beater manufactured by Valley Iron Works, Appleton, Wisconsin, for laboratory testing, and hand sheets were formed.

Bursting strength of the formed hand sheets was determined following the methods prescribed in TAPPI Standard Methods, No. T 220m5 3 and T-403-m5 3 using a Mullen tester manufactured by B. F. Perkins and Son, Inc., of Holyoke, Massachusetts.

Drainage characteristics of pulp samples taken from the Valley beater were checked using :a Schopper-Riegler f reeness tester, manufactured by Testing Machines, Inc.,

of New York City, and following the procedures outlinedin the TAPPI Standard Method No. T227m58 and TAPPI Data Sheet 20 The following table lists the drainage and bursting strength characteristics of a pulp prepared by the two stage method of this invention and a conventional magnesium-base sulfite pulp in relation to various beating times.

Values for Bursting Strength-lbs./sq. in./l00 lbs/24 x 36480 ream. Values for Freeness-cubic centimeters.

The commercial practices for paper manufacture require large scale continuous operation of paper-making equipment. Machine speed and thus, product output, is related to the rate with which the pulp slurry releases its water (i.'e., its drainage characteristics), which is indicated by the freeness measurement. A normal freeness for many operations is about 700 cc. based on the Sohopper- Riegler tester. From Table II above it isobserved that to reach this ifreeness with the pulp provided by the method of this invention thereis required about 30 minutes.

of beating time as compared to about 50 minutes required for pulp produced by the conventional magnesium-base sulfite method. This, of course, represents a substantial savings to the paper manufacturer in capital investment and operating costs of beater equipment.

In addition, it can be observed that for the equivalent.

freeness, the method of the present invention provides a pulp having greater strength characteristics as indicated by the respective Mullen test values of as against 116 at a freeness of about 700 cc., plus.

In the interests of clarity and the avoidance of repetition, the term sulfite liquor is used throughout the specification and in the claims as defining a bisulfite or soluble sulfite-bisulfite solution, and the term magnesium base refers to magnesium oxide or hydroxide.

Having thus described my invention and illustrated it by preferred embodiments, I claim as new and desire to pro tect by Letters Patent:

1. The method of manufacturing pulp from lignocellulose material by a two-stage magnesium-base sulfite digesting process which comprises adding suflicient additional magnesium base to a soluble magnesium base suliite liquor in the presence of said lignocellulose material in the digesting zone to provide an initial first-stage soluble cooking liquor having a molar ratio of sulfur dioxide to magneisum oxide in the range of 1.4 to 1.85 and a pH in the range of 5.9 to 5.0, said pH being obtained solely by said additional magnesium base.

2. The method of claim 1 wherein said initial first-stage soluble cooking liquor has a molar ratio in the range'of 1.6 to 1.7 and a pH in the range of 5.5 to 5.3, said pH being obtained solely by said additional magnesium base.

3. The method of manufacturing pulp from lignocelluose material by a two-stage sulfite pulping process which comprises adding sufficient additional magnesium base to a soluble magnesium-base sulfite liquor in the presence of said lignocellulose material in the digesting zone to provide an initial first-stage soluble cooking liquor having a molar ratio of sulfur dioxide to magnesium oxide in the range of 1.4 to 1.85, and an initial pH in the range of 5.9 to 5.0, said pH being obtained solely by said additional magnesium base, cooking said lignoeellulose material in the presence of said first-stage soluble cooking liquor until 15 to 30% of said lignocellulose material is dissolved, :removing most of said first-stagecooking liquor from said lignocellulose material, adding sulfur dioxide to the digester and cooking to complete the desired fiber liberation of said lignocellulose material.

4. The method of claim 3 wherein said initial first-stage soluble cooking liquor has a molar ratio in the range of being obtained solely by said additional magnesium base.

5. The method of claim 3 wherein said sulfur dioxide is added as an aqueous sulfur dioxide solution.

6. The method of claim 3 wherein said initial first-stage soluble cooking liquor has a molar ratio in the range of 1.6 to 1.7 and a pH in the range of 5.5 to 5.3, said pH being obtained solely by said additional magnesium base, and said sulfur dioxide is added as an aqueous sulfur dioxide solution.

7. The method of claim 3 wherein the amount of the cooking liquor removed at the end of the first-stage cook is in the range of from 60-65%.

References Cited in the file of this patent UNITED STATES PATENTS 2,032,437 Richter Mar. 3, 1936 2,749,241 Marpillero June 5, 1956 2,851,355 Battenberg Sept. 9, 1958 2,947,657 Peteri Aug. 2, 1960 FOREIGN PATENTS 207,134 Australia Mar. 20, 1957 

1. TTHE METHOD OF MANUFACTTURING PULP FROM LIGNOCELLULOSE MATERIAL BY A TTWO-STAGE MAGNESIUM-BASE SULFITE DIGESTING PROCESS WHICH COMPRISES ADDING SUFFICIENT ADDITIONAL MAGNESIUM BASE TO A SOLUBLE MAGNESIUM-BASE SULFITE LIQUOR IN THE PRESENCE OF SAID LIGNOCELLULOSE MATERIAL IN THE DIGESTING ZONE TO PROVIDE AN INITIAL FIRST-STAGE SOLUBLE COOKING LIQUOR HAVING A MOLAR RATIO OF SULFUR DIOXIDE TO MAGNEISUM OXIDE IN THE RANGE OF 1.4 TO 1.85 AND A PH IN THE RANGE OF 5.9 TO 5.0, SAID PH BEING OBTAINED SOLELY BY SAID ADDITIONAL MAGNESIUM BASE. 